Do you know how much he weighs now and what weight he should be? Your vets office should be able to look at his chart and tell you what weight he was before diabetes and if that is a good weight for him or if he needs to be a different weight. If he’s losing and is thin I echo strongly that he needs more food, regardless of what calorie count is that you are feeding him because he apparently isn’t getting what he needs from it. Inadequate calories contributes to his ketone issue that I know you’ve been battling. Triple check the calories of what you are feeding. A cat losing weight when he’s thin needs more calories no matter what the calorie count may be
I have great empathy—my boy has an adrenal tumor so regulation is aspirational at best, ketones are a constant struggle, and I’m constantly trying to fatten him up (unfortunately he only has an appetite when his BG is blue which is rare). And btw it took a long time to see the occasional blue. I cried for the first two months solid.
I agree with those who said to test his thyroid if not done (I’d like to think it was during his hospitalization): hyperthyroidism gives cats a ravenous appetite, causing weight loss and making it very difficult to control his diabetes. It’s very common in cats. Also check his kidney function—it can cause insulin resistance and is also very common in cats. Not uncommon in diabetic cats is acromegaly and very uncommon in cats is Cushing’s, both of which can cause insulin resistance. I’m not experienced enough to say whether a change in insulin is warranted, or testing for acromegaly or Cushing’s but I’d say ask your vet.
finally, and this is in no way a criticism, I noticed the first few days out of the hospital were his best BG days. Perhaps triple check your storage and handling and administration of insulin. I don’t use prozinc (I may have used the experimental form of it long ago)—I think it should be gently rolled but not shaken right? Check the expiration date also. Make sure you’re using the correct syringes ( I think prozinc uses the less common U40 right?) and drawing to the correct line.
as one cat mom of a diabetic ketotic cat to another —good luck!